Ceramic material

ABSTRACT

A ceramic material which comprises a composite of zirconia and O min -/ beta  min -sialon, preferably 5 to 30 volume percent of zirconia is a useful engineering ceramic material or advanced refractory material.

The present invention relates to an improved ceramic material and, inparticular, to an improved engineering ceramic material.

Engineering ceramics are materials such as the oxides, nitrides andcarbides of the metals silicon, aluminium, boron and zirconium. They arecharacterized by great strength and hardness; properties which in theorycan be retained to very high (>10000° C.) temperatures. Two of the mostpromising types of ceramic are the sialon family, and the zirconiafamily.

The sialons are based on the elements Si, Al, O, N, hence the acronym. Asuccessful commercial sialon is the β'-sialon which has the β-Si₃ N₄crystal structure, but with some of the silicon atoms replaced byaluminium atoms, and for valency balance some nitrogen atoms replaced byoxygen atoms. The sialons are usually formed by mixing Si₃ N₄, Al₂ O₃,AlN with a metal oxide (often Y₂ O₃), compacting the powder to thedesired shape, and then firing the component at ˜1750° C. for a fewhours. The function of the metal oxide is to react with the alumina andthe silica layer (which is always present on the surface of each siliconnitride particle), to form a liquid phase which dissolves the reactantsand precipitates the product. The liquid phase (which still containsdissolved nitrides), cools to form a glass between the β'-sialon grains.Typically, a Y₂ O₃ densified β'-sialon contains about 15 volume percentof Y-Si-Al-O-N glass and 85 volume percent β'-sialon. At temperaturesabove 800° C. this glass begins to soften and the strength decreases.The glass/sialon can be heat treated at ˜1300° C. to crystallise theglass. In the case of β'-sialon and glass, the glass crystallises togive Y₃ Al₅ O₁₂ (yttro garnet or YAG) and a small amount of additionalβ'-sialon. With glass/O'-sialon the crystallisation produces Y₂ Si₂ O₇(yttrium disilicate) plus a small amount of additional O'-sialon. Thiscrystallisation process reduces the room temperature strength of thematerial, but this reduced strength is maintained to higher temperature.The reason that crystallisation reduces strength is not completelyunderstood, but is probably because the crystalline YAG occupies asmaller volume than the glass it replaces; crystallisation leaves smallcracks. The grain boundary phase is a necessary evil in these materials,it is a remnant of the densification process.

β'-Sialon has the general composition Si_(6-Z) Al_(Z) P_(Z) N_(8-Z)where 0<Z≦4.2, whilst O'-sialon has the general composition Si_(2-X)Al_(X) O_(1+X) N_(2-X) where 0<X<0.20. O'-sialon has an expanded siliconoxynitride crystal lattice structure.

The β'-sialon is a strong engineering ceramic with good oxidationresistance and creep resistance up to 1300° C. The O'-sialon hasapproximately two thirds the strength of β'-sialon, but has very muchimproved oxidation resistance up to 1400° C. The two materials are inthermodynamic equilibrium and so composite materials can be formed. Thehigh temperature creep resistance is determined by the grain boundaryphase, which for these materials is usually YAG.

Another promising ceramic family is based on zirconia, ZrO₂. Themonoclinic or tetragonal zirconia is dispersed in a matrix typicallymullite, alumina or cubic zirconia. The dispersed zirconia toughens by aprocess known as transformation toughening. Basically, the composite isfired at high temperature (at least 1100° C.), when the ceramicdensifies, and the zirconia is in its high temperature tetragonal form.On cooling, part of the tetragonal zirconia attempts (and fails) totransform to its low temperature monoclinic form. The matrix constrainsthe zirconia in its tetragonal form which at room temperature ismetastable. This transformation would be accompanied by a 3-5 volumepercent increase in each zirconia crystal. The effect is to put theentire matrix into compressive stress, rather like prestressed concrete.Any crack running into such a ceramic tends to trigger the tetragonal tomonoclinic transformation which generates compressive stresses whichtend to close off the crack. The process becomes more efficient, thestiffer the matrix, because the stiff matrix is better able to constrainthe metastable tetragonal form at room temperature. The process is lesseffective at high temperature, and there is no toughening at all above900° C. because the tetragonal zirconia is now stable not metastable. Atoughening effect may also be obtained by the presence of dispersedmonoclinic zirconia because of microcracking effects.

Whilst it would be desirable to attempt to zirconia toughen sialonsbecause they are stiff (and hard and strong) but are also quite tough tostart with, workers in this field have found that zirconia reactschemically with β'-sialon and is partly reduced to zirconiumoxynitrides.

We have now surprisingly found that composites of β'-sialon and zirconiahave reduced amounts of stabilized cubic zirconia when O'-sialon isincluded therein. The present invention is based upon this discovery.

Accordingly, the present invention provides a ceramic material whichcomprises a composite of zirconia and O'-/β'-sialon.

The ceramic material of the invention may contain from 5 to 95 volumepercent zirconia based on the total volume of the composition.

The ceramic material may comprise a dispersion of zirconia in anO'-/β'-sialon matrix and such a dispersion is obtained when the amountof zirconia is from 5 to 30 volume percent, preferably from 10 to 25volume percent based on the total volume of the composition.

The volume ratio of O'-sialon to β'-sialon in the composites of theinvention may vary between wide limits but preferably will be in therange of from 1:7 to 7:1, more preferably in the range of from 1:3 to3:1.

The ceramic materials of the present invention may include in theO'-/β'-sialon matrix a solid solution of zirconia with yttria, ceria,lanthanum oxide, calcium oxide, magnesium oxide or a rare earth metaloxide.

The present invention furthermore provides a process for the preparationof a ceramic material as hereinbefore described which process comprisesthe reaction sintering at a temperature in the range of from 1500 ° to1750° C. of zircon, silicon nitride and alumina or a precursor foralumina, optionally in the presence of a reaction sintering aid or aprecursor therefor.

The primary function of the metal oxide sintering aid is to form a solidsolution with the zirconia. Thus, the sintering aid reacts initiallywith the alumina and the surface layer of silica on the silicon nitrideto form a transient liquid phase which dissolves the silicon nitride andthe zircon and from which the zirconia and the O'-/β'-sialonprecipitate.

The sintering aid used in this process may be, for example, yttria,ceria, lanthanum oxide, calcium oxide, magnesium oxide of a rare earthmetal oxide, or a precursor for one of these compounds. Thus, we havefound that, the alumina for the above described process and thesintering aid may be provided by the use of a spinel.

Preferred spinels for use in the process of the invention are those ofmagnesium or calcium with the compound of the formula MgAl₂ O₄ beingparticularly preferred for use.

The spinel is incorporated into the mixture which is sintered in anamount sufficient to provide the desired amount of aluminium in thefinal O'-/β'-sialon matrix. The spinel is thus preferably used in anamount of up to 10% by weight based on the weight of the zircon andsilicon nitride, preferably in an amount of from 6 to 8% by weight.

Other precursors of various of the components incorporated into themixture reaction sintered according to the above process may also beused. Thus, the ceramic materal of the invention comprising a dispersionof zirconia in an O'-/β'-sialon matrix may be prepared by reactionsintering a mixture of zircon, silicon nitride, a metal silicate andalumina.

The metal silicate may be, for example, a silicate of calcium, magnesiumor barium. It will be appreciated that on heating to sinteringtemperatures the metal silicate will react with some of the zircon andsilicon nitride to form a liquid phase which promotes reaction anddensification by a solution-precipitation mechanism. The oxides whichmay be used as sintering aids may also be provided by precursors such ascarbonates or bicarbonates which decompose to the oxide under thesintering conditions. For example calcium oxide and magnesium oxide assintering aids may be provided by calcium carbonate or magnesiumcarbonate respectively.

We have also found that instead of using zircon (ZrSiO₄) in the processas described above, a mixture of zirconia (ZrO₂) and silica (SiO₂) maybe used. This modification has the advantage that, whereas in zircon theratio of ZrO₂ to SiO₂ is fixed, it is possible to vary the ratio ofzirconia to silica, as required. This may, in some instances, beparticularly advantageous.

The present invention thus provides in a further aspect a process forthe preparation of a ceramic material comprising a composite of zirconiaand O'-/β'-sialon, which process comprises the reaction sintering at atemperature in the range of from 1500° to 1750° C. of a mixture ofzirconia, silica, silicon nitride and alumina or a precursor therefor,optionally in the presence of a reaction sintering aid or a precursortherefor.

The reaction sintering aid, or the precursor therefor, used in thisalternative embodiment of the invention is as hereinbefore described.Furthermore, the alumina for this process and the sintering aid may beprovided by the use of a compound, e.g. a spinel as hereinbeforedescribed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an X-ray diffraction trace for composition A of Example 1;

FIG. 2 is an X-ray diffraction trace for composition D of Example 1;

FIG. 3 is a scanning electron micrograph of composition D of Example 1;

FIG. 4 is a plot of the strength of the material of Example 2 as afunction of temperature; and

FIG. 5 illustrates the oxidation resistance of the composition ofExample 2 as compared to the composition of Example 3.

The present invention will be further described with reference to thefollowing Examples.

EXAMPLE 1

The following compositions were ball milled for 24 hours underisopropanol, using a 3 mm zirconia mixing media. The slurry was pandried and the powder isostatically pressed at 20,000 psi into billets.

The weight ratio of silicon nitride to zircon to alumina was keptapproximately constant, whilst the yttria content was varied from zeroto 6 percent by weight based on the zirconia. The billets were fired at1700° C. for five hours in a carbon element furnace with a two hour risetime. The ratio of O'- to β'-sialon to zirconia was kept constant at51:31:18 volume percent equivalent to 43:30:27 weight percent.

Composition A

    ______________________________________                                        Silicon nitride       52.0   g                                                Alumina               8.8    g                                                Zircon                39.2   g                                                ______________________________________                                    

The fired billet had a density of 3.35 g.cm⁻³ which is 92-95% of thetheoretical density of 3.52-3.62 g.cm⁻³.

Composition B

    ______________________________________                                        Silicon nitride                                                                              51.8   g                                                       Alumina        8.7    g                                                       Zircon         39.0   g                                                       Yttria         0.5    g (represents 2% by                                                           weight based on ZrO.sub.2)                              ______________________________________                                    

The fired billet had a density of 3.33 g.cm⁻³ which is 92-95% of thetheoretical density of 3.52-3.62 g.cm³.

Composition C

    ______________________________________                                        Silicon nitride                                                                              51.5   g                                                       Alumina        8.7    g                                                       Zircon         38.8   g                                                       Yttria         1.0    g (represents 4% by                                                           weight based on ZrO.sub.2)                              ______________________________________                                    

The fired billet had a density of 3.28 g.cm⁻³ which is 91-93% of thetheoretical density of 3.52-3.62 g.cm⁻³.

Composition D

    ______________________________________                                        Silicon nitride                                                                              51.2   g                                                       Alumina        8.6    g                                                       Zircon         38.6   g                                                       Yttria         1.6    g (represents 6% by                                                           weight based on ZrO.sub.2)                              ______________________________________                                    

This composition was bloated and thus represents the upper limit ofyttria addition possible for a firing temperature of 1700° C. It isprobable that larger amounts of yttria can be used, providing that thefiring temperature is reduced.

FIG. 1 is an X-ray diffraction trace for Composition A. The X-raydiffraction trace was taken with copper Kα radiation. The trace showsonly O'-sialon, β'-sialon and zirconia. There is a peak at 30° which isnitrogen stabilised cubic zirconia.

An X-ray diffraction trace for Composition D is shown in FIG. 2. Thisshows both monoclinic and tetragonal zirconia with O'-sialon andβ'-sialon. A scanning electron micrograph of the composition of thisexample at 2000 times magnification is shown in FIG. 3 and demonstratesthe absence of grain boundary glass.

EXAMPLE 2

Zircon (38.5 g), silicon nitride (51.2 g), alumina (8.7 g) and neodymiumoxide (1.6 g) were thoroughly mixed together and isostatically pressedat 20,000 psi. The billets were then fired at 1700° C. for 3 hours witha two hour rise time. The amount of neodymium oxide corresponds to 6percent by weight based on the weight of the zirconia).

The fired density was 3.33 g/cm³ which was 92-95% of the theoreticaldensity of 3.52-3.62 g.cm³.

The material had a room temperature strength of 336 MPa. The strength asa function of temperature is plotted in FIG. 4.

EXAMPLE 3 (COMPARATIVE)

Silicon nitride (60.1 g), alumina (10.1 g), silica (20.0 g) and yttria(9.8 g) were thoroughly mixed together and isostatically pressed at20,000 psi. The billets were then fired at 1700° C. for 3 hours with atwo hour rise time. The material comprised O'-sialon, β'-sialon and aglass. The material had a room temperature strength of 290 MPa. Thestrength as function of temperature is plotted in FIG. 4. This materialshows as reduced strength at 1100° and 1200° C. as compared to thematerial of Example 2.

FIG. 5 illustrates the oxidation resistance of the composition ofExample 2 as compared to the composition of Example 3. It can be seenfrom this Figure that the composite of zirconia and O'-/β'-sialon has avery much better oxidation resistance than the composite of thisExample.

EXAMPLE 4

The following composition was ball milled for 24 hours underisopropanol, using a 3 mm zirconia mixing media. The slurry was pandried and the powder isostatically pressed at 20,000 psi into billets.

    ______________________________________                                        Silicon nitride       58.9   g                                                Alumina               10.9   g                                                Silica                0.5    g                                                Zircon                29.8   g                                                Magnesium oxide       1.0    g                                                ______________________________________                                    

The billets were fired at 1700° C. for 1 hour under nitrogen gas in acarbon resistance furnace.

The fired billet had a density of 3.33 g.cm⁻³, which is 97-98% of thetheoretical density of 3.39-3.42 g.cm⁻³.

The weight ratio of O'- to β'-sialon to zirconia was 45:35:20.

X-ray diffraction analysis of the sample indicated from a peak at 30°that less than 2% of the zirconia was nitrogen stabilized cubiczirconia, the remainder of the zirconia was monoclinic.

EXAMPLE 5

The following composition was ball milled for 24 hours underisopropanol, using a 3 mm zirconia mixing media. The slurry was pandried and the powder isostatically pressed at 20,000 psi into billets.

    ______________________________________                                        Silicon nitride       55.2   g                                                Alumina               9.7    g                                                Silica                5.4    g                                                Zircon                29.7   g                                                Magnesium oxide       1.0    g                                                ______________________________________                                    

The billets were fired at 1700° C. for 1 hour under nitrogen gas in acarbon resistance furnace.

The fired billet had a density of 3.25 g.cm⁻³, which is 97-98% of thetheoretical density of 3.31-3.34 g.cm⁻³.

The weight ratio of O'- to β'-sialon to zirconia was 60:20:20.

X-ray diffraction analysis of the sample indicated from a peak at 30othat less than 2% of the zirconia was nitrogen stabilized cubiczirconia, the remainder of the zirconia was monoclinic.

What is claimed is
 1. A ceramic material which comprises a composite ofzirconia and O'-/β'-sialon, the volume ratio of O'-sialon to β'-sialonbeing in the range of 1:7 to 7:1.
 2. A ceramic material as claimed inclaim 1, which comprises from 5 to 95 volume percent of zirconia, basedon the total volume of the composition.
 3. A ceramic material as claimedin claim 1 which comprises a dispersion of zirconia in an O'-/β'-sialonmatrix.
 4. A ceramic material as claimed in claim 3, which comprisesfrom 5 to 30 volume percent of zirconia, based on the total volume ofthe composition.
 5. A ceramic material as claimed in claim 1, whereinthe composite includes therein a solid solution of zirconia with yttria,ceria, lanthanum oxide, calcium oxide, magnesium oxide or a rare earthmetal oxide.
 6. A ceramic material as claimed in claim 1, wherein thevolume ratio of O'-sialon to β'-sialon is in the range of from 1:3 to3:1.